feat(refboard): canvas polish — transform box sync, dot grid, snap tuning, perf fixes

- Fix transform box not updating after alignment/arrangement/normalize/flip operations
  (shortcuts, context menu, and selection toolbar all fixed with DRY _opUpdate helper)
- Replace 100ms polling loop with event-driven viewport updates (moved + wheel-scroll)
- Add adaptive dot grid background that responds to zoom/pan
- Reduce snap guide threshold from 8px to 4px for subtler snapping
- Remove PresenceOverlay (remote selection highlighting) — too heavy for minimal benefit
- Offset multiple dropped images so they don't overlap
- Add new canvas modules: SnapGuides, clipboard, grouping, FrameSprite, DrawingSprite,
  LaserPointer, context-menu-items, SelectionToolbar, Minimap
- Extract Editor hooks into dedicated files (useBoardLoader, useCanvasSetup,
  useShortcutHandler, useLayerPanel, useSaveManager, useFollowMode)
- Sync improvements: real-time transform broadcast, board rooms, viewport sync

frontend/src/canvas/shortcuts.ts

@@ -43,7 +43,10 @@ export interface ShortcutContext {
   handleUngroup: () => void;
   toggleGrid: () => void;
   toggleShowHelp: () => void;
+  toggleFocusMode: () => void;
+  fitSelection: () => void;
   writeCanvasToClipboard: (items?: SceneItem[]) => Promise<void>;
+  pasteFromSystemClipboard: () => Promise<string>;
 }
 
 /**